As the dimensions of circuit components become increasingly smaller, their number per unit area on the printed circuit board becomes even larger, accordingly, the soldering sites also become ever smaller. The component assembly on printed circuit boards--namely for the mass produced apparatus of entertainment electronics, telecommunications and computer chips--is carried out with very high accuracy by automated assembly machinery. The conducting strips then are connected by soldering to the terminals of the circuit elements. Various procedures are known for soldering.
In a known procedure of the above type, the printed circuit board is preheated and as a rule is brought into contact, along the path which rises somewhat relative to the horizontal, with the top area of a flowing soldering wave or even with two soldering waves. The direction of flow of the soldering wave may be in the same direction as, or opposite to, that of the advancing circuit board. Where two soldering waves are used, their directions of flow may be mutually opposite.
Typically, the assembled printed circuit boards are placed in controlled, displaceable, advance frames of which the path of advance is determined by a driven conveyance means. Such advance frames comprise insets which, in turn, are provided with clearances the shape of which accurately match the contour of the printed circuit board to be solder. To operate a soldering product on line or track, that is a finishing track comprising also a station for applying fluxing agents, a preheating station and lastly a wave soldering system, a plurality of advance frames with the corresponding insets will be required. Typically, it is attempted to insert rectangular printed circuit boards into typically rectangular insets in such a process, the two mutually opposite sides of the printed circuit boards are thus parallel to the direction of advance and thereby to the direction of flow of the soldering wave, when the former comes to make contact with the latter.
It has been found, however, that the quality of soldering in a printed circuit board depends on the configuration, that is the "geographic" arrangement, of the circuit elements assembly when the printed circuit comprises components (chips) on the side to be soldered. This assembly configuration now is quite unrelated to the contour of the printed circuit board, aside the obvious fact that the assembly must be within the contour.
Therefore, it has previously been suggested to orient the clearances in the insets of the advance frames at an angle to the direction of advance such that, for a specific configuration of component assembly, optimal soldering shall be achieved. Yet the magnitude of this angular deviation can be ascertained only empirically, whereby the optimal angle requires trying out many different insets, and then equipping all present advance frames with insets wherein the clearances provide the optimal angular offset.
The consequence is tradeoff in that the space within the advance frame is not being utilized optimally, that is, the advance frame is not provided with insets having the theoretically maximum number of clearances.
Not only is such a procedure quite time-consuming, but it also requires a great expenditure of effort, material and time to refit a soldering track.
Moreover, it was found that even for the case of two soldering wave machines with two mutually opposite soldering waves the soldering is not optimal because, manifestly, here again the result is highly dependent on the configuration of the assembly.
A prior art method, such as referenced in the preamble of the claim 1, can be construed as previously known from the German patent document No. A1 3,501,376. Therein, two sequentially mounted soldering waves are provided, and the printed circuit boards, mounted in the conventional insets of a conventional advance system, are consecutively brought into contact along a straight path with the top of the first and then the top of the second soldering wave. The soldering waves are arranged at a slant to the direction of advance of the advance system, whereby an acute angle results between the two mutually facing sides of the printed circuit boards and the direction of flow of the soldering shafts. While this acute angle lengthens the path along which the side of the printed circuit board to be soldered will be contacted, or "licked", by the top of the soldering wave, the side of the printed circuit board to be soldered will only be contacted from one side. Thus, in such a prior art approach, soldering sites following an elevated circuit element (chip), rising from the circuit board, will be in the "shadow" of this circuit element and therefore the soldering wave will not reach those soldering sites.
It is noted herein for the sake of completeness that it is further known to solder assembled printed circuit boards in the absence of advance frames. This is accomplished by a so-called "finger conveyor", wherein the printed circuit boards are seized at opposite edges by driven clamping fingers with barbs, the fingers being mounted on either side of the advance path to an endless, revolving and guided conveyor member. While advance frames are thus no longer required, a finger conveyor is designed only for a single format of printed circuit boards which are moved parallel to one of their sides over the soldering wave. Moreover, there is the danger of the printed circuit boards being bent during preheating and soldering.
Another soldering procedure with or without advance frames, is known as "reflow soldering" by those skilled in the art. Therein, the still unassembled printed circuit board is silkscreen printed with a paste containing solder and flux only at the future soldering sites, and thereafter the printed circuit board is assembled with components in an automated machine and, following this assembly, is heated, for instance by infrared, whereby the solder melts in the past and the conducting strips are connected by solder to the terminals of the components. However, reflow-soldering has hardly found any acceptance in practice, probably in part because extraordinary accuracy and coincidence are absolutely necessary both in printing and assembling.